Determination of resistance sources is essential for development of varieties resistant to biotic and abiotic stresses. Germplasm evaluation can offer valuable breeding materaials which result in different tolerant combinations. In this study, 47 sugar beet monogerm o-type lines (male strile maintainer) along with two susceptible and two resistant checks were evaluated for resistance to Cercospora leaf spot in in vitro conditions using leaf disk method. The test was conducted based on a completely randomized design with three replications. In each replication, 10 leaf disks from 10 plants were checked for every genotype. The leaf disks were sprayed with fungal spores in in vitro conditions and after 8 days the number of spots was counted in leaf area index. Based on the results, most of the genotypes showed susceptibility, but 5 monogerm lines had no significant difference with resistant check. The selected genotypes along with three susceptible genotypes, a susceptible check and a resistant check were evaluated in natural infected conditions. The trial was carried out using to a completely randomized block design with four replicates. Plots were scored three times according to a 1-9 scale and the mean of four replications was used as the score of each genotype every time. According to the results, the susceptible genotypes recevived high score and were susceptible in the field condition. From 5 selected genotypes, only one line acted in a similar way to resistant check and had no significant difference with it.